Why does this matter? The city has a closed primary system, which means that you can only vote in the primary for the party you chose when you registered. However, a lot of people signed up with no party, which means they won't be able to vote in the primary. For Ward One, for example, there are three Democrats and one Republican running, and mayoral candidates Adrian Fenty and Vince Gray are Democrats as well -- so if you want to vote, you had better check your affiliation, as you can't change it when you get to the polling place if you were already registered. However, if you are registering for the first time at the polling place, you can.
The city's Board of Elections has links on the left side for both checking and changing your registration status, so check it out. Thanks to both DCist and the Ward One listserve for the heads-up.